Muography hit the headlines recently as the technique used by a team of researchers to discover that the Great Pyramid of Giza has an extra two chambers than originally thought. Of all the analytical techniques employed by scientists, it is one of less widely-known techniques and is only known due to that single use.
Muography is a non-destructive imaging technique that uses muons to distinguish between regions of high density and low density to create a projectional image. Muography is used for large structures (natural or man-made) that contain different (and distinct) internal regions and it largely benefits from a wide effective detection range and a high spatial resolution.
Muography utilizes a muon beam that fires elementary (subatomic) particles known as muons (leptons with ½ spin and a – 1 e charge) through a target object. It is very similar in nature to radiography – which uses the same process but with X-rays instead of muons.
The beam of muons passes through a target’s external surface/wall to analyze and identify inaccessible components by their differences in density. In low density structures, the muons do not interact, stop or decay unlike in higher density structures.
The difference in interaction potential with an internal structure enables a distinction between low and high-density regions to occur, as more muons travel through the low-density regions compared to the high-density regions.
This difference in detected muons (and their trajectories) is picked up by sensors and compared. An image of the internal features of a structure can then be constructed by converting the produced muogram into a muograph. The detectors are often made of materials that are sensitive to charged particles (from either a chemical or electronic perspective) with nuclear emulsions (a photographic plate with an emulsion layer and uniform grain size) being a common choice.
Applications of Muography
Muography has, and can be, used on a wide variety of natural and man-made structures. Aside from the Pyramids in Egypt, researchers have also used muography techniques on the pyramids at Teotihuacan, Mexico.
In the man-made world, muography has recently been used to inspect nuclear reactors. Fukushima is the most notorious example and muography was employed to locate the nuclear fuel after it was damaged in a 9.1 magnitude earthquake.
Muography is most commonly used in the geophysical sector and is especially useful for mapping the internal structure of large-scale geological features, namely volcanoes. Muography has been used to map the internal structures of volcanoes pre- and post-eruption, and is currently used to collect data on the magma flows within volcanoes to help better predict when eruptions are likely to occur.
Muography is currently being used at Vesuvius, Stromboli, Etna and Puy de Dôme. Muography is also used across other geological applications, namely in the monitoring of groundwater, particularly in areas susceptible to landslides and heavy rainfall.
The Future of Muography
Whilst muography use is currently contained to the geology of the Earth, this may not be true in the future. Due to its long-distance imaging capabilities, muography may also be employed in the future to image the geological features of other planetary bodies.
Cosmic rays are in abundance in outer space. It is deemed entirely possible that the primary cosmic rays could interact with the Earth’s atmosphere to generate pions/mesons, which subsequently change into muons upon decay. This process is also thought to be possible with other planetary atmosphere’s and could provide a way for more in-depth imaging of local planetary bodies.
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NASA is also working on some projects to help distinguish the internal regions of smaller celestial bodies (100-1000 m diameter) using muons produced from charged mesons, as well as using K,π muons to image the surface features of bodies 10-100 m in diameter.
Due to the scale of the imaging, the different atmospheric conditions and the lower muon flux, the equipment is set to be different to those seen on earth to accommodate for these factors, but the basic principles will remain the same.
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Sources:
BBC News: http://www.bbc.co.uk/news/science-environment-41845445
University of Tokyo: http://www.eri.u-tokyo.ac.jp/people/ht/MUOGRAPHY/techniques.html
NASA: https://www.nasa.gov/sites/default/files/files/Prettyman_NIAC_Symposium_2014.pdf
“Development of nuclear emulsion for muography”- Morishima K., et al, Annals of Geophysics, 2017, doi:10.4401/ag-7387
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